Models were a part of the Quick Menu during Early Access. Now it is simply a catch-all reference to anything that can be built or placed or found in the world.
Items are obtained from Treasure Chests and various Quests and even from collecting them from broken Objects in the world. Items consist of a variety of melee and ranged weapons as well as various useful items. Weapons greatly increase the combat abilities of the Player by enabling ranged combat and granting greater damage. Many Items are used to complete Quests in the game.
Characters are randomly found throughout the world in every Biome. Friendly and neutral characters are unlocked by completing Quests. Aggressive and Hostile Characters must be defeated in combat to be unlocked. Once unlocked, these new Characters must still be purchased in order to change to that Character or to place it into the world. Unlocking a Character also unlocks all of its parts for use in the Character Customizer.
Creatures are randomly found in the world. Most are unlocked automatically or by completing a Quest for them. After unlocking they can be bought and afterwards spawned as often as wanted. Many creatures can be mounted and ridden around the landscape, through the seas or through the skies.
Vehicles are found randomly generated in nearly every Biome and can be unlocked using the Discovery Tool. No quest is needed to be completed to Discover a vehicle, but it must still be unlocked by paying Studs. After purchase Vehicles can be spawned an unlimited time for free. Some vehicles are discovered through special Codes.
See: Brick Builds
Brick Builds are a set of complex pre-defined structures, many of which are also found randomly generated in the world. A few Brick Builds are available right away and they do not need to be purchased, but the rest must be discovered through Treasure Chests, completing Quests or by downloading them through the Build Showcase. Original Brick Builds can either be imported from a build system like the LEGO Digital Designer or by using the in-game tools to build your own then selecting and copying it with the Copy Tool.
Objects are all things that can be found in the world: furniture, plants, decorations, etc. They are easily discovered using the Discovery Tool but must be still be purchased before they are able to be placed in the world. Several Quests involve placing one or more Objects within a specified area.